Google says its Guetzli compression produces JPEG images with fewer photo-degrading artifacts. These zoomed-in images show the original at left, Guetzli compression in the middle, and an alternative called libjpeg at right.

Google

It's a common problem: A website's text is smudged with stray pixels. It's as much fun as seeing white dog hairs on the black couch.

The culprit is the JPEG file format, which can compress graphics so they load faster on your PC and eat up less of your phone's monthly data plan. There have been lots of efforts to do better -- Microsoft's JPEG XR and Google's WebP and RAISR among them -- but their success has been limited by the ubiquity of JPEG support.

Smaller file sizes might seem an arcane technology concern, but they're crucial to fast-loading websites. The average web page has ballooned from about 1 megabyte five years ago to about 2.5MB today, according to the HTTP Archive, and bigger pages load slowly. The faster the web page, the happier everyone is: Speed means we buy more online, read more news pages and spend more time checking friends' social network activity.

For Google's Guetzli speed boost, researchers developed a test called Butteraugli designed to model human vision. Compression works by throwing out data that we won't notice is missing, and the point of Butteraugli is to automate testing of different compression settings. Guetzli fiddles with two particular parts of JPEG compression -- discrete cosine transform, which governs how details like object edges are recorded, and quantization, which governs which colors are preserved and which are sacrificed to cut file size.

Note how the Guetzli compression on the right is smoother but lacks some richer colors of the libjpeg compression in the center. The original image is on the left.

Google

There's no free lunch here, though. Guetzli may indeed produce better perceived quality at a given file size, but note for example how some green areas are washed out in the eye comparison image above. And although Google compared Guetzli to mozjpeg and another JPEG encoder called libjpeg, there are other options, too.

Another problem: speed of compressing images. "Guetzli is rather slow to encode," the researchers said, suggesting it's most likely useful on image-heavy websites. "Although Guetzli may be too slow for many practical uses, we hope that it can show direction for future image format design," the researchers said.

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